1. Field of the Invention
The invention relates to a power converter, and more particularly to a method and apparatus for synchronous rectifying of soft switching power converters.
2. Description of the Related Art
FIG. 1 shows a circuit schematic of a soft switching power converter. It includes a transformer 10 to provide isolation from AC line input to the output of the power converter for safety. The soft switching of the power converter achieves high efficiency and low EMI (electric-magnetic interference) performance. Transistors 20 and 30 develop a half bridge circuit to switch a resonant tank and the transformer 10. The resonant tank is formed by an inductor 15 and a capacitor 40. The inductor 15 acts as a primary-side leakage inductance of the transformer 10 and/or an inductance device. The inductance L of the inductor 15 and the capacitance C of the capacitor 40 determine the resonance frequency f0.
                              f          0                =                  1                      2            ⁢            π            ⁢                                          L                ×                C                                                                        (        1        )            
The transformer 10 transfers the energy from the primary side to the secondary side of the transformer 10. Rectifiers 41 and 42 rectify the switching voltage of the transformer 10 into a capacitor 65. A DC voltage VO is thus generated at the output terminal of the power converter.
The synchronous rectifier on the secondary side of the transformer is a further approach to achieve a higher efficiency power conversion, such as “Control circuit associated with saturable inductor operated as synchronous rectifier forward power converter” by Yang, U.S. Pat. No. 7,173,835. However, the disadvantage of this prior art is an additional power consumptions caused by saturable inductors, etc. The object of present invention is to provide a method and apparatus for the synchronous rectifying of soft switching power converter to achieve higher efficiency.